The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.
An imaging lens for mounting in such a small camera is strongly required to have a small size, and also sufficient optical performances that can be compatible with imaging elements with high resolution that are available in these years. Conventionally, when resolution of an imaging lens was not so high, it was possible to attain both sufficient optical performances suitable for the resolution of the imaging element and miniaturization. However, as resolution of an imaging lens becomes higher, required optical performances become higher, so that it becomes difficult to attain both sufficient optical performances with satisfactorily corrected aberration and miniaturization using a two- or three-lens configuration.
For this reason, there have been studies on adding another lens, i.e., an imaging lens with a four-lens configuration. For example, an imaging lens described in Patent Reference includes in this order from an object side, a first lens that has a convex shape on the object side and is positive; a second lens that has a shape of a negative meniscus lens directing a convex surface thereof to the object side; a third lens that has a shape of a negative meniscus lens directing a concave surface thereof to the object side; and a fourth lens that has a negative meniscus lens directing a convex surface thereof to the object side. According to this configuration, satisfactory optical performances are obtained while restraining increase of a total length of the imaging lens by setting preferred ranges for a ratio of a focal length of the imaging lens and a focal length of the third lens and for a ratio of a distance on an optical axis from an object-side surface of the first lens to an image-side surface of the fourth lens and the focal length of the imaging lens, and then respectively keeping those ratios within the ranges.
Patent Reference: Japanese Patent Publication No. 2009-98513
According to the imaging lens described in Patent Reference, it is possible to attain relatively satisfactory aberrations. Every year, however, there have been advancements in miniaturization and performances of devices themselves for mounting in the above-described small cameras, so that the size required for such imaging lens has become even smaller than before. In case of the lens configuration described in Patent Reference, it is difficult to attain both miniaturization and satisfactory aberration correction to meet the above-described requirements.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberration in spite of a small size thereof.